1. Field of the Invention
The present invention generally relates to a system and method of driving a load having a cyclically variable input requirement. More particularly, but not exclusively, the invention relates to the synchronization of the output characteristic of an electrical machine with the load characteristic of the apparatus to which it is coupled.
2. Description of Related Art
Many types of rotating electrical machines are known for coupling to mechanical equipment, either to act as motors and thus drive the equipment by supplying mechanical power to it, or to act as generators and thus be driven by the equipment by drawing mechanical power from it. General descriptions and classifications of these types of electrical machines can be found in many standard textbooks, e.g. Chapter 10 of Newnes Electrical Engineer's Handbook, edited by D. F. Warne and published by Butterworth-Heineman 2000, incorporated herein by reference. In general, the requirements for coupling the machine and the mechanical equipment center on the quality of alignment of their shafts, i.e. the installer seeks to minimize any radial or parallax errors between the axes of the shafts. Consideration is not normally given to the angular (i.e. the rotational) alignment of the shafts.
One such type of machine is the switched reluctance machine which, in recent years, has returned to prominence as a type to be considered for variable-speed applications. The general theory of the design and operation of switched reluctance machines is well known and is discussed, for example, in “The Characteristics, Design and Applications of Switched Reluctance Motors and Drives”, by Stephenson and Blake and presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993 and incorporated herein by reference.
The switched reluctance machine is generally constructed without windings or permanent magnets on the moving part (generally called the rotor). The stationary part of most switched reluctance machines (called the stator) includes coils wound around stator poles that generally carry unidirectional current. In one type of switched reluctance motor, coils around opposing stator poles are connected in series or parallel to form one phase winding of a potentially multi-phase switched reluctance machine. Motoring torque is produced by applying a voltage across each of the phase windings in a predetermined sequence that is synchronized with the angular position of the rotor so that a magnetic force of attraction results between poles of the rotor and stator as they approach each other. Similarly, generating action is produced by positioning the pulse of voltage in the part of the cycle where the poles are moving away from each other. In typical operation, each time a phase winding of the switched reluctance machine is energized, magnetic flux is produced by the phase winding, thereby causing a force of attraction on the rotor poles.
The most common form of switched reluctance machines are rotary and cylindrical, with an internal rotor. However, inverted, linear and segmented machines are also known. Embodiments of the invention are applicable to the different types of machine.
In order to maintain the torque and related speed developed by a switched reluctance machine, it is desirable to control carefully the instants at which voltage is applied to the phase windings of the motor. A wide variety of control schemes exists and some of these schemes are discussed in the Stephenson paper cited above.
Different types of electrical machines have characteristically different profiles of torque as a function of angle, and it is often these characteristics (known or perceived) which influence a designer when selecting an electrical machine to fulfil a particular duty. For example, it is traditionally held that DC machines have a particularly smooth torque/angle profile (i.e. they have low torque ripple) and these machines are often chosen for, e.g., driving the rolls in a steel mill or driving the traverse of a grinding machine in the expectation that the lack of torque ripple will enable the production of a high quality product. By contrast, it is known that the single-phase induction motor has a very high torque ripple (since the torque is produced by a pulsating, rather than a rotating field), so it would not be considered a good choice for applications where, say, the load had a low inertia and was sensitive to torque ripple.
One of the criticisms which is frequently levelled at the switched reluctance machine is that it has inherently high torque ripple which can only be suppressed at the expense of degrading the magnitude of the average torque or uprating the power converter to handle higher currents than would otherwise be required. In the opinion of many researchers, this is a major weakness of the switched reluctance machine and precludes it from the wider application it might otherwise enjoy. For example, the paper “Torque ripple minimization in switched reluctance motor drives by PWM current control” by Husain and Ehsani, Proc of IEEE 9th Applied Power Electronics Conference, Orlando, Fla. Feb. 13-17 1994, pp 72-77, incorporated herein by reference, contains a detailed discussion of the mechanism of torque ripple and describes the development of one method of reducing it. It has been accepted up to now that the concept of torque ripple involves a variable output which has to be reduced. However, attempts to minimize torque ripple according to known methods typically also reduce the average torque available from the motor.